Matches come from the tags Steam players vote on each game, weighted so a rare tag counts for far more than one that sits on half the store. A game's popularity never moves it up this list.
The closest matches to 7 Billion Humans on Steam are Human Resource Machine, TIS-100, SHENZHEN I/O, Turing Complete, SpaceChem. They rank highest because their Steam community tags overlap most with 7 Billion Humans's — shared tags include Programming, Education, Puzzle.
How are games like 7 Billion Humans chosen?
Every game is described by the tags Steam players vote on it. Rare, specific tags such as Programming, Education, Puzzle count far more than catch-all ones like "Indie" or "Singleplayer", and each game is scored on how closely its whole tag profile matches 7 Billion Humans's. Popularity never affects the ranking.